U.S. Pat. No. 387,184 shows that in 1888 it was known to utilize two relatively longitudinally reciprocating flat dies, each with a tapered portion to gradually begin the thread rolling and then each with a dwell portion which was parallel to the longitudinal direction of movement which completed the thread rolling and gave a finish form to the threads. This type of machine is still used today wherein there is no provision for tilt of the dies or the die holders in a longitudinal direction, and it is intended that the dwell portion shall be parallel to the longitudinal relative direction of movement of the die holders in order to provide the finish form rolling to the threads.
The German Pat. No. 294,822 showed that in 1914 it was known to use reversible dies. Machinists had learned between 1888 and 1914 that if one were going to the expense of heat treating the dies and then shipping and selling them to a customer, the customer might as well get double life from the die by having it reversible face-to-face and using the second die face for a threading operation. However, because of the high forces encountered in the thread rolling, the dies need to be backed by a supporting surface in the die holder, so in this second type of machine the die holder pockets are somehow adjustable for tilt in a longitudinal direction. This is so that the thread rolling on the workpiece is gradually increased from the entrance end to the exit end. However, this type of tilt-adjustable die holder pocket has the disadvantage that the flat contact surface of the die ridges which form the threads on a workpiece cannot have any parallel dwell portion for precisely finishing the thread form.
U.S. Pat. No. 1,232,876 shows a different form of reversible die, being reversible end-for-end rather than face-to-face, because the length of the workpiece is far shorter than the width of the die, so the die can be made to accommodate thread forms to roll form workpieces of two different lengths.
U.S. Pat. No. 1,972,225 is a 1934 patent which acknowledges it was customary to make the thread rolling dies reversible, and shows them reversible face-to-face. These are relatively thick dies which will withstand the force of the thread rolling, and in order to make the dies reversible and yet the thread ridges on the back face of the die supported in the die holder, the second type of machine would be used wherein the entire die pocket would be tilted longitudinally relative to the longitudinal direction of movement.
Other patents such as U.S. Pat. No. 2,314,391 appear to show the die pockets parallel to the direction of movement, with the thread rolling face of the die at a slight angle to the longitudinal direction of movement.
U.S. Pat. No. 2,825,251 illustrates the first type of machine wherein the die pockets are parallel to the direction of movement yet each thread forming face in the die has a tapered portion and a parallel dwell portion.
The issued patents support the practice in the industry, namely, that since at least 1888 it has been known to use die holders which are not adjustable for longitudinal tilt, with both dies having a tapered portion and a parallel dwell portion for finishing the thread forms. It has been known since at least 1914 to increase the effective die life by making the dies reversible face-to-face, but in order to support the die against the high forces of thread rolling, the reversible die contact surface was necessarily flat to engage a flat planar surface in the die holder pocket, and then the entire die holder pocket was tiltable longitudinally.
This means that the depth of penetration of the thread rolling action continues completely to the very end of the movable die. There is often a very short tapered roll-off portion to prevent marking the threaded workpiece; however, without any parallel-to-motion dwell portion, the threaded workpiece is often slightly oval in form in order to have enough rolling pressure to get the crests of the thread forms completely filled out. With this ovaliform cross section to the workpiece, it is more difficult to keep the workpieces within specification as to minimum and maximum diameters, and as a result more time is spent in keeping the machine in proper adjustment in order to produce workpieces within specification.